Polymerizations and Condensations
Reactions forming new bonds were summarized as “polymerization and condensation reactions”. The term polymerization included the formation of dimers as it is the case in [2+2]-cycloaddition reactions of unsaturated compounds.
Additional substrates undergoing this kind of transformation have been reported by Ciamician and Silber in reactions demanding skill and patience. The dimerization of stilbene 65 in solution was reported after two years and four months of irradiation. A coumarin dimer 68 was found whose syn head-to head structure was finally proven in 1962 (Scheme 17).[99]
49a–c 50a–c a: R1 = R2 = H
b: R1 = Me, R2 = H c: R1 = iPr, R2 = Me
51a–c 52a–c
53a–c 54a–c
55a–b 56 57a–b 58 59a–b
60 61 62 63 64
65 66
67 68 Scheme 16: -Cleavage of carbonyls: Norrish Type I and II reaction.
Scheme 17: Intermolecular [2+2]-cycloaddition reactions in solution.
Besides intermolecular [2+2]-cycloadditions, the first intramolecular reaction was found in case of the monoterpene ketone carvone 69, which gave an isomeric product in a clean reaction, whose structure could not be determined exactly by Ciamician and Silber. They proposed the tricyclic product carvonecamphor 70 to be formed. The proposal was found to be correct in 1957 (Scheme 18).[100]
A similar reaction including the [2+2]-cycloaddition of an olefin and a carbonyl group giving oxetanes 73 was discovered by another important Italian pioneer of photochemistry, Emanuele Paternò (1847 – 1935), in Rome in 1909.[101] The reaction was improved in 1954 by the Swiss-American chemist George Hermann Büchi (1921 – 1998) and is today known as the Paternò–Büchi reaction (Scheme
19).[102]
Another reaction of synthetic importance was discovered irradiating 2-butanone 74. Beside the reduction product 2-butanol 75, an interesting con-densation product as a result of oxidative dimerization was identified:
3,4-dimethylhexane-2,5-dione 76. This 1,4-diketone could be used for the synthesis of pyrrol derivatives (Scheme 20).
69 70
71a–c 72 73a–c
74 75 76 Scheme 18: Intramolecular [2+2]-cycloaddition of carvone to carvonecamphor.
Scheme 19: Paternò-Büchi reaction
Scheme 20: Radical dimerization of methylethylketone.
: [2+2]-cycloaddition of carbonyls and olefins.
Scheme 20: Radical dimerization of methylethylketone.
Autooxidations
In addition to oxidation reactions caused by irradiation of carbonyls, a variety of light-induced oxidations involving molecular oxygen have been treated. In some cases, different products were found in presence or absence of molecular oxygen.
In the above-mentioned -cleavage of acetone, formic acid 79 was found instead of methane when oxygen was present in the reaction vessel. Highly substituted aliphatic ketones gave tertiary alcohols 82. Cyclic ketones were oxidatively cleaved giving carboxylic and dicarboxylic acids 84 and 85 under aerobic conditions.
2-Substituted cyclic ketones yielded ketoacids 87 who themselves underwent a second cleavage, resulting in the corresponding carboxylic acids 88 and 89.
Alkylated aromatic compounds such as toluene were oxidized to benzaldehyde 91 and benzoic acid 92 whereas in the absence of oxygen the dimer 1,2-diphenyl-ethane was obtained. Finally, stilbene 93 was converted to benzaldehyde in the presence of light and oxygen. The mechanism was explained by addition of an activated oxygen molecule to the double bond of stilbene forming a dioxetane intermediate followed by cleavage of the four-membered ring (Scheme 21).
77 78 79
80 81 82
83 84 85
86 87 88 89
Rearrangements
Finally, unsaturated compounds were studied in consideration of their behavior towards light. Geometric isomerization was known to occur in olefins. Additionally, Ciamician and Silber studied similar isomerizations of carbon-nitrogen- and nitrogen-nitrogen double bonds such as the syn-anti isomerization of oximes 95 and the isomerization of cyanoazo derivatives 97 (Scheme 22).[94]
The synthetic progress of light-driven reactions was still accompanied by a lack of mechanistic understanding. Ciamician tried to give his work an interdisciplinary character by combining aspects of theoretical chemistry, physical chemistry, and biology. However, physics made great progress at the beginning of the twentieth century. Johannes Stark (1874 – 1957) and Albert Einstein (1879 – 1955) formulated the second law of photochemistry between 1908 and 1913, which is also known as quantum equivalence law.[55] It says that in primary photochemical processes, each absorbed quantum of radiation causes one equivalent of a chemical reaction. Nevertheless, a deep mechanistic and theoretical understanding of Ciamician’s and Silber’s photoreactions was still not possible. The theory about electronically excited states was yet to come. From today’s point of view, the mechanistic background of all reactions involving carbonyls, for example, is the
93 94
95 96
97 98 Scheme 21: Autooxidation reactions.
Scheme 22: Syn-anti isomerization of oximes and cyanoazo derivatives.
high reactivity of their triplet state with regard to fast hydrogen abstraction generating C-centered radicals.[93]
The value of Ciamician’s and Silber’s work is not only described by the numerous new photoreactions they discovered, but also by Ciamician’s attempt to establish photochemistry as a science of its own. He summarized all the work known so far.[1] Moreover, in 1912, he drew a visionary picture about future applications of light-driven reactions.[2] Emphasizing the interdisciplinary character of his work he found parallels with biology. “For plants, light is the source of energy. Through the intervention of chlorophyll, green plants accumulate solar energy and transform it into chemical energy … Chemistry in the laboratory differs from chemistry of organisms not in the materials, but in the reagents used. It is thus apparent that the further advancement of biology requires that all of the compounds present in nature can be produced by using only reagents present in nature, rather than agents that do not belong to the living world.”[1, 93] An understanding of chemical reactions in organisms was seen to be of high importance for the progress of man-made chemistry.[103] In organic synthesis, photochemistry was seen to be able to imitate the mild conditions of biochemical reactions.[94] Photochemistry could be used for the synthesis of fine chemicals.
Special attention was paid to reactions leading to natural products, as could be shown in the synthesis of sugars or fatty acids. Moreover, it was realized that complex structures like the tricyclic compound carvonecamphor 70 could be photochemically generated in one step out of simple structures (carvone 69).
Ciamician found several examples where light as a mild agent served as a susbstitute for strong reagents and harsh conditions. The selective oxidation of alcohols to aldehydes by benzoquinone was seen as an alternative for strong
Additionally, Ciamician drew the picture of a photochemical industry. In times where coal was the backbone of the industry he recognized fossil fuels not being
inexhaustible whereas light energy offered a lasting supply to industrialization.[93]
He already predicted solar home heating, photo-electric batteries and the production of solar fuels.[2, 58] In a way, this kind of chemistry refers to what is today called “green” or “sustainable chemistry,” although the terms were proposed later. But as we know, his request to exploit solar energy as the only lasting form of energy came too early. The visionary picture of “The Photochemistry of the Future”
from 1912 did not find its breakthrough in industry, but its ideas were shared by other scientists. The German chemist Hans Stobbe (1860 – 1938) for example, was also campaigning for a bigger role of photochemistry. He is best known for his pioneering studies on the photochromism of fulgides 99, a light-induced color change of organic dyes.[104, 105]
The beginning of the twentieth century remains as a time of great progress in photochemistry since many types of photoreactions were already known. Besides Ciamician’s and Silber’s summary of photochemical reactions, several books appeared. In 1912, Alfred Benrath (1871 – 1969) from Königsberg (Germany) published a “Lehrbuch der Photochemie” (Textbook of Photochemistry).[106] Ivan Plotnikov (1878 – 1955), a Russian photochemist, wrote several books, for example, “Photochemische Versuchstechnik” (1911), “Grundriss der Photochemie”, and “Lehrbuch der allgemeinen Photochemie.”[107] He gave a summary of the development of photochemistry where he noticed a “pre-scientific period up to about 1850” and a “second period of lively advancement” mainly combined with the development of photography. According to him, no earlier than at the beginning of the twentieth century had photochemistry begun its full development.[94]
99 100 Scheme 23: Photochromism of fulgides.
1.6 Developments in the Twentieth Century
World War I marked a break in many respects. A number of research activities had come to a sudden end. Many excellent results had to be rediscovered. Later on, many researchers continued in the field of organic photochemistry. Among them should be named only a few: Günther Otto Schenck (1913–2003) discovered the Schenck reaction, which is a photosensitized diastereoselective ene reaction of alkenes 101 with singlet oxygen that affords allylic hydroperoxides 102 (Figure
7).[108]
g
He is also known for his work on the theory of photosensitization and chemical engineering in the field of solar photochemistry.[109] In 1944, he used chlorophyll from spinach leaves as photosensitizers for the selective oxidation of -terpinene 103 to produce ascaridol 104 via a cycloaddition of oxygen and the diene.[110] This drug was urgently needed at that time as an anthelmintic to fight ascaris infections in humans. To address this, Schenck constructed an open-air solar irradiation pilot plant for the methylene blue-sensitized production of ascaridol (Scheme 24). Later, he became involved in the technical development of water disinfection by UV irradiation.[109]
g K. Schaffner, Günther Otto Schenck (1913-2003): A Pioneer of Radiation Chemistry. Angew. Chem.
Int. Ed. 2003, 42, 2932-2933. DOI: 10.1002/anie.200390509. Copyright © 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Reproduced with permission.
Figure 7: Günther Otto Schenck.
Figure 7: Günther Otto Schenk and his pilot plant for the production of ascaridol in his garden in Heidelberg (1949).g
The area of solid-state photochemistry that Liebermann had opened was enlarged by Gerhard Martin Julius Schmidt (1919 – 1971), a German scientist working at the Weizmann Institute in Israel who was familiar with the fields of organic chemistry, crystallography, and spectroscopy (Figure 8). This constellation led to the discovery of “topochemistry.” The term means that solid-state reactions are determined by the geometry of the reactant lattice. In other words, reactions in the solid state occur with a minimum amount of atomic or molecular movement.
[111, 112]
h
h D. Ginsburg, Gerhard M. J. Schmidt 1919-1971. Israel Journal of Chemistry. 1972, 10, 59-72.
Copyright © 1972 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Reproduced with permission.
101 102
103 104
Figure 8: Gerhard M. J. Schmidt, a pioneer of solid-state photochemistry. h
Scheme 24: Schenck-ene reaction and synthesis of ascaridol.
Figure 8: Gerhard M. J. Schmidt, a pioneer of solid-state photochemistry.h
Another scientist of international significance is the German Alexander Schönberg (1892 – 1985).[113] He emigrated to Egypt in 1937 due to unsafe political circumstances in Germany (Figure 9). In Egypt, his work on solar photochemistry was favored by reason of an annual availability of 3500 hours of sunshine.[114] Besides many photochemical reactions he described the photochemical epoxidation of a carbonyl group in methanol[115] and the addition of olefins to o-quinones to give 1,4-dioxenes, which is today known as “Schönberg addition.”[116]
But he is most famous for his book “Preparative Organic Photochemistry”.[117]
This book is the last portrait of photochemistry as it was before the theory of electronically excited states came up.[94] It uses an order different from that of today’s textbooks. Nowadays, textbooks show a theoretically based classification.
It is an interesting fact that many photochemical reactions were discovered although the underlying physical principles of all these processes were unexplored. Early photochemists did not know about the nature of light and its effect on organic molecules to create electronically excited states. The development of physical chemistry and spectroscopy, particularly time-resolved spectroscopy, made concepts like quantization of energy, electronically excited singlet and triplet states and deactivation of excited states via electron transfer available, which could explain the mechanisms of many photoreactions.[55, 118]
105 106
107 108 109 Scheme 25: Photochemical epoxidation of carbonyls and Schönberg addition.
i
All pioneers of photochemistry have created the basis for today’s research concerning light-mediated chemical transformations. Today, the need of an improved disposal of solar energy is reflected in the discussion about “green chemistry” and the implementation of renewable energies. Maybe someday Ciamician’s hundred-year-old vision of the exploitation of solar energy will come true: “And if in a distant future the supply of coal will become completely exhausted, civilization will not be checked by that, for life and civilization will continue as long as the sun shines.” [2]
i Reprinted with permission from: E. Singer, Alexander Schönberg 1892-1985. Chem. Ber. 1987, 120, I-XIX. Copyright © 1987 Deutsche Bunsen-Gesellschaft für physikalische Chemie e.V.
Figure 9: Photochemistry in Egypt: Alexander Schönberg. i
1.7 Key Steps in Early Organic Photochemistry
Table 1: Important steps in photochemistry.
Name Year Contribution to Photochemistry
Joseph Priestley 1790 First reported photoreaction Nicholas de Saussure 1804 Principles of photosynthesis Humphry Davy
Johann W. Döbereiner 1831 Photoreduction of metal salts Carl Julius Fritzsche 1867 Dimerization of anthracene
Carl T. Liebermann 1877 First [2+2]-cycloaddition; artificial light sources Arthur Downes
Thomas Blunt 1879 Photochemical radical formation William Henry Perkin
Johannes Wislicenus 1881 Geometric Isomerization
Julian Schramm 1884 Light-induced halogenation of alkylbenzenes Heinrich Klinger 1886 Photoreduction of carbonyls; photochemistry for
synthetic purpose
Bertram and Kürsten 1895 Dimerization of styrene derivatives
Hermann Trommsdorff 1834 Photorearrangement of santonin; wavelength dependency of photoreactions
Paul Silber 1900 Systematic study of the behavior of light towards matter
Hans Stobbe 1908 photochromism of fulgides Johannes Stark
Emanuele Paternò George H. Büchi
1909
1954 Paternò-Büchi reaction Ivan Plotnikov
Alfred Benrath
1911
1912 Textbooks on photochemistry Ronald G. W. Norrish 1930s -cleavage of carbonyls in gas phase Günther Otto Schenck 1940s Photosensitization; Schenck-ene-reaction Gerhard M. J. Schmidt 1960s Solid-state photochemistry; topochemistry Alexander Schönberg 1968 New photoreactions; textbook “Preparative
Organic Photochemistry”
1.8 References
[1] G. Ciamician, Sur Les Actions Chimique De La Lumière. Bull. Soc. Chim.
France 1908, 4, i.
[2] G. Ciamician, The Photochemistry of the Future. Science 1912, 36, 385-394.
[3] H. D. Roth, The Beginnings of Organic Photochemistry. Angew. Chem., Int. Ed.
Engl. 1989, 28, 1193-1207.
[4] J. Priestley, Experiments and Observations on Different Kinds of Air, Birmingham, 1790.
[5] A. J. Ihde, The Development of Modern Chemistry, New York, 1964.
[6] H. Remane, W. Girnus, Meilensteine der Chemie 2012. Nachr. Chem. 2012, 60, 11-21.
[7] M. Faraday, On Two New Compounds of Chlorine and Carbon, and on a New Compound of Iodine, Carbon, and Hydrogen. Phil. Trans. R. Soc. Lond. 1821, 111, 47-74.
[8] T. Tidwell, Sunlight and free radicals. Nature Chem. 2013, 5, 637-639.
[9] J. W. Draper, Text-Book on Chemistry, Harper&Brothers, New York, 1861.
[10] H. Remane, W. Girnus, Meilensteine der Chemie 2014. Nachr. Chem. 2014, 62, 12-23.
[11] F. Arago, Fixation des images qui se forment au foyer d'une chambre obscure. Comptes Rendus 1839, 8, 4-7.
[12] F. Arago, Le Daguerreotype. Comptes Rendus 1839, 9, 250-257.
[13] J. F. Döbereiner, S. J., Pharm. Centralbl. 2 1831, 383-385.
[14] J. Fritzsche, Über die festen Kohlenwasserstoffe des Steinkohlentheers. J.
Prakt. Chemie 1867, 101, 333-343.
[15] K. Elbs, Über Paranthracen. J. Prakt. Chemie 1891, 44, 467-469.
[16] C. Liebermann, Über Polythymochinon. Ber. Dtsch. Chem. Ges. 1877, 10, 2177-2179.
[17] J. Yamamoto, R. Martin, S. Iwai, P. Plaza, K. Brettel, Repair of the (6-4) Photoproduct by DNA Photolyase Requires Two Photons. Angew. Chem., Int.
Ed. 2013, 52, 7432-7436.
[18] J. Bertram, R. Kürsten, Über das Vorkommen des Orthocumaraldehyd-methyläthers im Cassiaöl. J. Prakt. Chemie 1895, 51, 316-325.
[19] C. Liebermann, Über Cinnamylidenmalonsäure und die stereoisomeren Cinnamylidenessigsäuren (Phenylpentadiensäuren). Ber. Dtsch. Chem. Ges.
1895, 28, 1438-1443.
[20] C. Liebermann, Über die Umlagerung der Allofurfurakrylsäure und Allocinnamylidenessigsäure im Sonnenlicht. Ber. Dtsch. Chem. Ges. 1895, 28, 1443-1448.
[21] C. N. Riiber, Die Synthese der α-Truxillsäure. Ber. Dtsch. Chem. Ges. 1902, 35, 2411-2415.
[22] A. Downes, T. P. Blunt, Researches on the Effect of Light upon Bacteria and other Organisms. Proc. R. Soc. Lond. 1877, 26, 488-500.
[23] A. Downes, T. P. Blunt, The Effect of Sunlight Upon Hydrogen Peroxide.
Nature 1879, 20, 521-521.
[24] W. H. Perkin, On the isomeric acids obtained from coumarin and the ethers of hydride of salicyl. J. Chem. Soc., Trans. 1881, 39, 409-452.
[25] J. Wislicenus, Ber. Verh. Sächs. Akad. Wiss. Leipzig, Math.-Naturwiss. Kl. 1895, 489-493.
[26] L. Stryer, J. Tymoczko, J. Berg, Biochemie, Fifth ed., Spektrum Akademischer Verlag GmbH, Heidelberg, 2003.
[27] J. Schramm, Über den Einfluss des Lichtes auf den Verlauf chemischer Reactionen bei der Einwirkung der Halogene auf aromatische Verbindungen. Monatshefte für Chemie 1888, 9, 842-853.
[28] J. Schramm, Ueber die Bildung des Parabrombenzylbromids durch Einwirkung von Brom auf Parabromtoluol, ohne Mitwirkung der Wärme.
Ber. Dtsch. Chem. Ges. 1884, 17, 2922-2925.
[29] J. Schramm, Ueber den Einfluss des Lichtes auf den Verlauf chemischer Reaktionen beim Bromiren aromatischer Verbindungen. Ber. Dtsch. Chem.
Ges. 1885, 18, 350-355.
[30] J. Schramm, Ueber den Einfluss des Lichtes auf den Verlauf chemischer Reaktionen bei der Einwirkung der Halogene auf aromatische Verbindungen. Ber. Dtsch. Chem. Ges. 1885, 18, 606-609.
[31] J. Schramm, Ueber den Einfluss des Lichtes auf den Verlauf chemischer Reaktionen bei der Einwirkung der Halogene auf aromatische Verbindungen. Ber. Dtsch. Chem. Ges. 1885, 18, 1272-1279.
[32] J. Schramm, Ueber den Einfluss des Lichtes auf den Verlauf chemischer Reactionen bei der Einwirkung der Halogene auf aromatische Verbindungen. Ber. Dtsch. Chem. Ges. 1886, 19, 212-218.
[33] H. Klinger, Ueber das Isobenzil und die Einwirkung des Sonnenlichts auf einige organische Substanzen. Ber. Dtsch. Chem. Ges. 1886, 19, 1862-1870.
[34] H. Klinger, Ueber die Einwirkung des Sonnenlichts auf organische Verbindungen. Justus Liebigs Annalen der Chemie 1888, 249, 137-146.
[35] H. Klinger, W. Kolvenbach, Die Bildung von Acetohydrochinon aus Acetaldehyd und Benzochinon im Sonnenlicht. Ber. Dtsch. Chem. Ges. 1898, 31, 1214-1216.
[36] H. Klinger, O. Standke, Ueber die Einwirkung des Sonnenlichts auf organische Verbindungen. Ber. Dtsch. Chem. Ges. 1891, 24, 1340-1346.
[37] H. Klinger, Über Synthesen durch Sonnenlicht. Justus Liebigs Annalen der Chemie 1911, 382, 211-221.
[38] H. Trommsdorff, Über Santonin. Ann. Chem. Pharm. 1834, 11, 190-207.
[39] T. v. Grotthuß, Physisch-chemische Forschungen, Schrag, Nürnberg, 1820.
[40] F. Sestini, Gazz. Chim. Ital. 1876, 6, 357-369.
[41] F. Sestini, Gazz. Chim. Ital. 1879, 9, 298-304.
[42] F. Sestini, L. Danesi, Gazz. Chim. Ital. 1882, 12, 82-83.
[43] S. Cannizzaro, F. Sestini, Ricerche sulla santonina. Gazz. Chim. Ital. 1873, 3, 241-251.
[44] H. Schiff, Ber. Dtsch. Chem. Ges. 1876, 9, 1689-1692.
[45] S. Cannizzaro, Gazz. Chim. Ital. 1876, 6, 341-348.
[46] S. Cannizzaro, Gazz. Chim. Ital. 1876, 6, 355–356.
[47] V. Villavecchia, Ueber einige Derivate des Santonins. Ber. Dtsch. Chem. Ges.
1885, 18, 2859-2864.
[48] S. Cannizzaro, G. Fabris, Über eine neue vom Santonin abstammende Säure (Isophotosantonsäure). Ber. Dtsch. Chem. Ges. 1886, 19, 2260-2265.
[49] H. D. Roth, A tribute to Stanislao Cannizzaro, chemical informationist and photochemist. Photochem. Photobiol. Sci. 2011, 10, 1849.
[50] G. R. Clemo, R. D. Haworth, E. Walton, The constitution of santonin. Part I.
The synthesis of dl-santonous acid. J. Chem. Soc. (Resumed) 1929, 2368-2387.
[51] E. E. van Tamelen, S. H. Levin, G. Brenner, J. Wolinsky, P. Aldrich, The Structure of Photosantonic Acid. J. Am. Chem. Soc. 1958, 80, 501-502.
[52] E. E. van Tamelen, S. H. Levin, G. Brenner, J. Wolinsky, P. E. Aldrich, The Structure of Photosantonic Acid. J. Am. Chem. Soc. 1959, 81, 1666-1678.
[53] O. L. Chapman, L. F. Englert, A Mechanistically Significant Intermediate in the Lumisantonin to Photosantonic Acid Conversion. J. Am. Chem. Soc. 1963, 85, 3028-3029.
[54] M. H. Fisch, J. H. Richards, The Mechanism of the Photoconversion of Santonin. J. Am. Chem. Soc. 1963, 85, 3029-3030.
[55] H. D. Roth, Twentieth century developments in photochemistry. Brief historical sketches. Pure Appl. Chem. 2001, 73, 395-403.
[56] G. Ciamician, Gazz. Chim. Ital. 1886, 16, 111-112.
[57] G. Ciamician, P. Silber, Über die Einwirkung des Lichtes auf eine alkoholische Nitrobenzollösung. Ber. Dtsch. Chem. Ges. 1886, 19, 2899-2900.
[58] N. D. Heindel, M. A. Pfau, A profitable partnership: Giacomo Ciamician and Paul Silber. J. Chem. Educ. 1965, 42, 383.
[59] G. Ciamician, P. Silber, Chemische Lichtwirkungen. Ber. Dtsch. Chem. Ges.
1900, 33, 2911-2913.
[60] G. Ciamician, P. Silber, Chemische Lichtwirkungen I. Ber. Dtsch. Chem. Ges.
1901, 34, 1530-1543.
[61] G. Ciamician, P. Silber, Chemische Lichtwirkungen II. Ber. Dtsch. Chem. Ges.
1901, 34, 2040-2046.
[62] G. Ciamician, P. Silber, Chemische Lichtwirkungen III. Ber. Dtsch. Chem. Ges.
1902, 35, 1992-2000.
[63] G. Ciamician, P. Silber, Chemische Lichtwirkungen IV. Ber. Dtsch. Chem. Ges.
1902, 35, 3593-3598.
[64] G. Ciamician, P. Silber, Chemische Lichtwirkungen V. Ber. Dtsch. Chem. Ges.
1902, 35, 4128-4131.
[65] G. Ciamician, P. Silber, Chemische Lichtwirkungen VI. Ber. Dtsch. Chem. Ges.
1903, 36, 1575-1583.
[66] G. Ciamician, P. Silber, Chemische Lichtwirkungen VII. Ber. Dtsch. Chem. Ges.
1903, 36, 4266-4272.
[67] G. Ciamician, P. Silber, Chemische Lichtwirkungen VIII. Ber. Dtsch. Chem. Ges.
1905, 38, 1176-1184.
[68] G. Ciamician, P. Silber, Chemische Lichtwirkungen IX. Ber. Dtsch. Chem. Ges.
1905, 38, 1671-1675.
[69] G. Ciamician, P. Silber, Chemische Lichtwirkungen X. Ber. Dtsch. Chem. Ges.
1905, 38, 3813-3824.
[70] G. Ciamician, P. Silber, Chemische Lichtwirkungen XI. Ber. Dtsch. Chem. Ges.
1907, 40, 2415-2424.
[71] G. Ciamician, P. Silber, Chemische Lichtwirkungen XII. Ber. Dtsch. Chem. Ges.
1908, 41, 1071-1080.
[72] G. Ciamician, P. Silber, Chemische Lichtwirkungen XIII. Ber. Dtsch. Chem. Ges.
1908, 41, 1928-1935.
[73] G. Ciamician, P. Silber, Chemische Lichtwirkungen XIV. Ber. Dtsch. Chem. Ges.
1909, 42, 1386-1391.
[74] G. Ciamician, P. Silber, Chemische Lichtwirkungen XV. Ber. Dtsch. Chem. Ges.
1909, 42, 1510-1515.
[75] G. Ciamician, P. Silber, Chemische Lichtwirkungen XVI. Ber. Dtsch. Chem. Ges.
1910, 43, 945-949.
[76] G. Ciamician, P. Silber, Chemische Lichtwirkungen XVII. Ber. Dtsch. Chem.
Ges. 1910, 43, 1340-1350.
[77] G. Ciamician, P. Silber, Chemische Lichtwirkungen XVIII. Ber. Dtsch. Chem.
Ges. 1910, 43, 1536-1541.
[78] G. Ciamician, P. Silber, Chemische Lichtwirkungen XIX. Ber. Dtsch. Chem. Ges.
1911, 44, 1280-1289.
[79] G. Ciamician, P. Silber, Chemische Lichtwirkungen XX. Ber. Dtsch. Chem. Ges.
1911, 44, 1554-1558.
[80] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXI. Ber. Dtsch. Chem. Ges.
1911, 44, 1558-1564.
[81] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXII: Autooxydationen I.
Ber. Dtsch. Chem. Ges. 1912, 45, 38-43.
[82] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXIII. Ber. Dtsch. Chem.
Ges. 1912, 45, 1540-1546.
[83] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXIV: Autooxydationen II.
Ber. Dtsch. Chem. Ges. 1912, 45, 1842-1845.
[84] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXV: Autooxydationen III.
Ber. Dtsch. Chem. Ges. 1913, 46, 417-422.
[85] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXVI: Autooxydationen IV. Ber. Dtsch. Chem. Ges. 1913, 46, 1558-1565.
[86] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXVII: Autooxydationen V. Ber. Dtsch. Chem. Ges. 1913, 46, 3077-3084.
[87] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXVIII: Autooxydationen VI. Ber. Dtsch. Chem. Ges. 1913, 46, 3894-3899.
[88] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXIX: Autooxydationen VII. Ber. Dtsch. Chem. Ges. 1914, 47, 640-646.
[89] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXX. Ber. Dtsch. Chem.
Ges. 1914, 47, 1806-1814.
[90] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXXI: Autooxydationen VIII. Ber. Dtsch. Chem. Ges. 1915, 48, 181-187.
[91] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXXII. Ber. Dtsch. Chem.
Ges. 1915, 48, 187-190.
[92] G. Ciamician, P. Silber, Chemische Lichtwirkungen XXXIII. Ber. Dtsch. Chem.
Ges. 1915, 48, 190-195.
[93] A. Albini, M. Fagnoni, Green chemistry and photochemistry were born at the same time. Green Chem. 2004, 6, 1.
[94] A. Albini, V. Dichiarante, The ‘belle époque’ of photochemistry. Photochem.
Photobiol. Sci. 2009, 8, 248.
[95] R. G. W. Norrish, F. W. Kirkbride, Primary photochemical processes. Part I.
The decomposition of formaldehyde. J. Chem. Soc. (Resumed) 1932,
The decomposition of formaldehyde. J. Chem. Soc. (Resumed) 1932,